This invention relates to an oscillation circuit in which the oscillation frequency varies in response to the intensity of light incident thereon, and more particularly to an oscillation circuit for controlling a bootstrap circuit included in an EPROM (Erasable Programable Read Only Memory) which is erasable by means of ultraviolet rays.
In general, a package for mounting an EPROM which is erasable by means of ultraviolet rays is provided with a window for allowing ultraviolet rays to be radiated on the EPROM. During erasing of the stored contents, ultraviolet rays are radiated on the EPROM through the window. However, during writing of data in the EPROM, external light rays may also penetrate through the window to radiate on the EPROM. These external light rays incident on junction capacitances in the bootstrap circuit included in the EPROM may cause leakage of charges stored in the junction capacitances, resulting in the write-in voltage being lowered to a level insufficient to perform the function of writing-in the data. In order to avoid this lowering of the write-in voltage, an oscillation circuit is usually included in the EPROM so that the junction capacitances are repeatedly charged up during writing.
In a conventional oscillation circuit the oscillation frequency is constant regardless of the intensity of the incident light rays; but here there is a disadvantage in that the oscillation frequency may be too high or to low when the incident light rays are relatively feeble or relatively intense, respectively. More particularly, the high potential H and the low potential L are alternately repeated at the output of the oscillation circuit. Each of the high potential levels corresponds to a writing interval. Each of the low potential levels corresponds to a reset interval for charging up the junction capacitances. If the frequency is too high, the sum of the reset intervals becomes too large, so that the writing time is restricted so that data cannot be written in the EPROM. On the contrary, if the frequency is too low, the charges are depleted, due to the external incident light rays as mentioned above, from the junction capacitances, so that the charging-up of the junction capacitances during each reset interval is not sufficient to maintain a voltage high enough to write data in the EPROM, because each writing interval is too long. In both cases, write-in efficiency is deteriorated.